Hybrid electric vehicles (HEV) can selectively utilize an internal combustion engine and one or more high-voltage motor generator units (MGU) as alternate or concurrent power sources to optimize fuel efficiency. That is, an HEV having a full hybrid powertrain can be propelled using purely electrical means at least some of the time, usually immediately upon starting the HEV and when operating below threshold vehicle speeds. One or more MGUs may alternately draw power from and deliver power to an energy storage system (ESS) as needed. Upon launch of the vehicle or when operating above a threshold speed, the engine can be restarted using the MGU or an auxiliary starter motor, depending on the vehicle design, and then engaged with a transmission to propel the vehicle via a set of drive wheels.
Hybrid powertrains typically share various fuel saving features, e.g., regenerative braking and automatic start-stop functionality. Regenerative braking allows for selective recharging of the ESS using the MGU(s). The capability of an HEV to selectively shut off and restart its engine when at a standstill, and/or when operating in a stabilized low-speed drive mode, likewise provides a fuel-saving benefit relative to conventional vehicle designs.